1. Field of the Invention
The present invention relates to a developing device for use in electrophotographic copy machines, and more specifically relates to a developing device that produces a developed image by having a thin layer of a monocomponent developing material comprising only a toner make contact with an electrostatic latent image formed on the surface of a photoconductive member.
2. Description of the Prior Art
Conventional monocomponent developing devices provide concavo-convexities on the surface of a developing sleeve to improve toner transportability and transportable quantity to the surface of said developing sleeve as well as to sufficiently impart a charge to said toner.
A monocomponent developing device having a developing sleeve provided a roughened surface thereon, said roughened surface being accomplished by sandpaper or the like on the surface of said developing sleeve is disclosed in U.S. patent application No. 4,377,332. However, when the surface of a developing sleeve is roughened by providing sandpaper or the like on the surface thereof, the concavo-convexities are sharp and cause damage to the photoconductive member through friction, thereby reducing the durability of the device.
On the other hand, Japanese Patent Application No. 63-241579 discloses a developing device providing a fiber-like filler material of glass fiber or the like having a particle diameter of 5 to 30 .mu.m. A fiber-like filler material is distributed inside the developing sleeve so as to protrude from the surface of said developing sleeve, as shown in FIG. 4, the surface of said developing sleeve having concavo-convexities formed thereon by the combination of the protruding portions of the fiber-like material and non-protruding portions of the fiber-like material. The aforesaid concavo-convexities on the surface of the developing sleeve are also sharp, and therefore are unsuitable for use in contact-type monocomponent developing devices. Further, the toner readily fuses to the concavities due to the sharp edges of said concavities. A characteristic of the aforesaid invention is that it assumes that the developing sleeve becomes worn with use and the resulting wear gradually will expose the fiber-like filler material distributed inside the developing sleeve at the surface of said sleeve so as to maintain the concavo-convexities formed on the surface thereof. However, when the surface of the developing sleeve becomes worn, the fiber-like filler material particles protruding from the surface of the developing sleeve break off from the sleeve. When the broken off fiber-like filler material mixes with the toner, said broken off fiber-like filler material not only affects the chargability of the toner and the toner transporting power but also causes damage to the photoconductive member, sleeve and toner thin-layer forming member when said fiber becomes packed between the toner thin-layer forming member and the developing sleeve because the broken off fiber-like filler material has a particle diameter that is greater than the diameter of the toner particles.
Monocomponent developing methods require that the electrical resisticity of the developing sleeve surface be controllable so as to be maintained within a specified range. When the electrical resistivity is less than 10.sup.6 .OMEGA./cm, image density gradation characteristics deteriorate, and the reproducibility of ultrafine and high-density halftone dots is reduced. Further, when pin hole defects exist in the photoconductive member, or when a discharge is produced from the end of the developing sleeve, an extremely large electric field is generated between the grounded photoconductive member and the developing sleeve and produces a developing bias voltage leak that reduces the developing bias, thereby causing uneven density, grainy fogging, or image dislocation; said voltage leakage may damage the photoconductive member. On the other hand, when the electrical resistivity is greater than 10.sup.14 .OMEGA./cm, density gradation is excellent but image density is inadequate causing deterioration in fine line and halftone dot reproducibility.
Thus, the addition of carbon black, lead oxide or similar powder-like microparticle material has been conventionally suggested as a means of maintaining the adjustment of electrical resistance within the previously described range. However, the aforesaid type of conductive particles generally exhibit a strong cohesive force between electrical conductive particles, making it difficult to achieve uniform dispersion of the particles in the developing sleeve. Moreover, conductivity cannot be imparted unless a large quantity of conductive particles are added because the developing sleeve is made conductive through contact among the particles. However, when a large quantity of conductive microparticles are added, the electrical resistance value is precipitously reduced, making it difficult to control the electrical resistance of the semiconductive region of 10.sup.6 to 10.sup.14 .OMEGA.cm required for the developing sleeve.
When a large quantity of conductive microparticles having inherently poor dispersibility are added as previously described, the dispersion of the electrical resistance within the developing sleeve becomes even greater and causes problems from the standpoint of image quality. Further, conventional conductive microparticles, of which carbon black is a representative example, cause a reduction in electrical resistivity under conditions of high temperature and high humidity because their hydrophilic functional groups are at the surface.
Still further, the strength of the developing sleeve itself is reduced and its wear resistance properties deteriorate when a large quantity of conductive microparticles is added.